First of all, here are two rules to understand the Pokemon anime :
1) If you watched the dub, you did NOT truly watch the anime.
2) Each Pokemon "cycle" is a different style of anime : different narrative, and different art style.
Pokemon Diamond&Pearl is an excellent anime. It has deep stories, it has great fights, and it has even less fillers than Naruto or Bleach.
DP starts by introducing Hikari, who wants to become a Top Coordinator as her mother was. But as she starts her journey, she'll realize she may not have the guts to succeed. Indeed, she will often fail, which will make her wonder if she is fit to be a Coordinator.
Ash and Pikachu, of course, come back as the MCs. Ash still aims to win a Pokemon League, only this time, he has a lot of experience: in this series, Ash does feel strong. He knows what he's doing, doesn't do much mistakes, knows how to raise his Pokemon.
However, he will be facing someone even more skilled as him: Shinji, who quickly appears to be an utter assh*le, even with his own Pokemon. Through the whole season, Ash and Shinji will fight whenever they come across each other. But everytime they clash, they learn from each other, and with time they learn to respect each other despite each's flaws.
Aside from that, the story has a perfect narrative: it has a lot of different stories (Ash's League, Hikari's Contests, Team Galactic's plot, and plenty others), and each story progresses constantly, which makes them interesting to follow.
But more importantly, the series is deep. It develops a very philosophical theme : the question of identity. Yes, you heard right.
Because all the characters (including human AND Pokemon) have a story that revolves around the idea of discovering one's self. Ash and Shinji wonder what a true Trainer is ; Hikari wonders if she's fit for the role of Coordinator ; Brock (finally) wonders what job he should take ; Pikachu is once again faced with the idea of evolving to become stronger ; ...
ALL the characters' stories go in the same direction: they are all faced with an identity crisis.
Pokemon DP didn't start really good. It was better than the previous Pokemon anime, but still not very good. Luckily, in the middle, they went HD ; starting from then, it became way more amazing.
The animation was cool, although not excellent. At least it was dynamic, which made for very good fights.
The OST in the original version is awesome. It takes the themes of the games, but make an orchestral version of them, which sounds way better.
It also creates its own themes, which are very cool.
The openings are also great, especially the first one, which I can't even describe, it's just... unique.
The characters are what makes this series stand out.
Ash has finally become an expert in Pokemon. While he does do some mistakes, they are now very rare, and he makes up for it with brillant and creative tactics.
Shinji was an absolute delight, as he is a complete assh*le, which makes him really stand out as a character in this show. He is also god damn powerful, which makes him the perfect rival.
While Ash makes friendship a priority, Shinji only cares about winning and doesn't care about his Pokemon. Two opposite philosophies. Of course, Shinji will learn that he is wrong, right?
WELL, NO! Ash and Shinji's relationship is called a DUALITY: their visions, although opposite, are actually complementary.
While they can't stand each other because of their opposite points of view, each will learn from the other and each will grow stronger thanks to the other.
Long story short, their relationship was so damn interesting to follow!
Aside from these two, I also loved Hikari. The fact that she keeps losing in Contests makes her really interesting, as she has a lot of room for progress (and she does progress across this series).
Pokemon DP is an absolute delight. This show is deep (RIP to all those who couldn't even understand that), the characters are very interesting (especially Shinji and his relationship with Ash), the fights were very cool (the Sinnoh League is full of memorable battles), the show is very serious and not that cheesy.